Tuesday, March 11, 2008

Nickel Brighteners (Class I)

The first brighteners of this class were the alkyl naphthalene sulfonic acids proposed. The commercial grades of these compounds were impure. The impurities resulted from the synthetic methods used in their large scale preparation, and materials as such caused brittle plate and were never used in practical nickel plating. These compounds were wetting agents and plates obtained were free from pitting. Benzene and naphthalene di- and trisulfonic acids was introduced without constituent groups, and these were successfully used commercially. They were far better than materials such as gum tragacanth and similar critical that only could be used for very thin deposits; otherwise the internal stress in the plate together with its brittleness would cause cracked, peeled plate. Sulfonic acids were not critical in concentration and did not caused poor adhesion or cracked plate with thick deposits; however, good brightness may be obtained on buffed surfaces and even then it left much to be desired. Unsaturated aliphatic sulfon compounds such as methyl, ethyl, propyl, or butyl or butyl sulfonic acids were developed.

These compounds are generally classified as class I nickel brighteners because they are sulfon compunds which like the plain aromatic sulfon compounds decrease tensile stress and introduce similar concentrations of sulfide sulfur into the plate. However, they also introduced carbonaceous material into the plate and give improve brightness. Later, propargyl sulfonic acid and 2-butylene-1,4-disulfonic acid were developed.

The unsatureted aliphatic sulfonic acids listed above as well as o-benzoyl sulfinamide, ortho- or metasulfobenzaldehyde, p-vinyl benzene sulfonic acid and pyridine-3-sulfonic acid give bright nickel plate in the presence of high concentrations of chloride (over 150 g/l NiCl2.6 H2O). Plain benzene and naphthalene sulfonic acids do not give bright nickel plate in these high-chloride baths.

Benzene sulfonic acids are more effective than the benzene or naphthalene sulfonic acids and benzene sulfonamides or sulfonimides in increasing the tolerance of the bath, Watts or high chloride, to excess of inorganic impurities, or excess of class II brighteners that cause poorly adherent, peeled, or dark plate or misplating in low-current density areas. The sulfinic acids undergo oxidation by air to the sulfonic acids.

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